The invention relates to a traveling wave ultrasonic motor, in particular, a motor that produces a traveling wave-like deformation on the surface of a stator and a rotor held elastically in contact with the stator for the rotational driving thereof by the traveling wave produced on the stator.
Such motors are known from U.S. Pat. No. 5,648,696, the content of which is incorporated by reference, and U.S. Pat. No. 5,726,519, the content of which is incorporated by reference. These piezoelectric motors are the result of research aimed at simplifying the mode of excitation proposed previously in the U.S. Pat. No. 4,562,374, the content of which is incorporated by reference, which describes for the first time a motor employing traveling wave drive through contact between a rotor and an annular stator in which the set of points of the contact surface is what gives rise to an elliptic motion and in which the creation of a traveling wave in the stator is achieved by applying the stator to a piezoelectric ceramic annulus working in the d31 mode. The simplification is in replacing the piezoelectric ceramic annulus by bar-shaped exciters using the longitudinal vibration mode or d33 mode, exhibiting better performance than the transverse mode d31. Furthermore, the exciters can consist of standard ceramic bars that are much less expensive to make than the flat annulae with alternating polarization. A structure which works in a similar manner is also described in the patent EP 0 569 673, the content of which is incorporated by reference.
The structure described in the U.S. Pat. No. 5,648,696 is composed of two coaxial rotors associated with two stators mounted between the rotors and of dual-polarization ceramic bars disposed between the stators and whose ends are respectively linked to each of the stators. This symmetric structure is advantageously used to benefit naturally from a nodal plane forming a fixed reference and allowing the fixing of the motor to the frame and the supplying of the transducers by means of a connector disposed in this nodal plane, as described in the U.S. Pat. No. 5,828,158, the content of which is incorporated by reference. At first sight, these structures appear to be practical and easy to implement. However, they in fact exhibit a major drawback, in that even when starting from geometrically perfect pieces, it is not possible to circumvent deformations of the stator annulus during the operation of static prestressing of the transducers. The structure described in the patent EP 0 569 673 seems to be less subject to this type of deformation, owing to the solid nature of the central core used, but this ignores the effect of amplification sought by the inventors, which amplifying effect applies equally to the undesirable static deformations and to the dynamic deformations. The amplitude of these static deformations may greatly exceed that of the traveling wave, so that one is very far from the contact conditions achieved in the case of excitation by geometrically uniform ceramic annulus bonded to the stator without prestress. Compensation for these defects requires sanding of the pieces after assembly and clamping of the structure, which operation is poorly suited to a simple process and renders subsequent surface treatment of said pieces problematic. Noncompensation for these defects may turn out to be catastrophic in terms of performance. If one considers, for example, a structure of the type described in the U.S. Pat. No. 6,093,994, the content of which is incorporated by reference, having, for each stator, four exciter elements, so as to produce therein a traveling wave of order 3 (three wavelengths), if one of the exciters is longer than the others, by 10 xcexcm for example, the static deformation produced is far greater than the amplitude of the traveling wave which is a few xcexcm. As a result the rotor/stator contact occurs only in the zone situated above the bar: the contact is driving over a positive half-wave and resistive over a negative half-wave, so that the resultant torque is zero and the motor does not turn. This shows that it is difficult, within industrial manufacture, to remedy this defect.
Even in the case where these geometrical defects are compensated for perfectly, a new effect will limit the performance of such longitudinal-excitation motors. This is the superimposing of radial vibrations on the normal and tangential vibrations produced in the stator. The system for the central prestressing of the ceramics is the origin of the radial component of vibration. This effect is described in the Yasuo patent, which seeks in particular to profit therefrom by inclining the zone of contact between stator and rotor. In this case, it is indeed the superimposing of the vibratory speeds which intervenes.
However, the consequence of the fact that the traveling wave is of multiple order is that, at any instant, several diametrically opposed zones of the rotor, in contact with the peaks of the traveling wave, are simultaneously subjected to antagonist radial forces. In order not to lose as much (or even more) energy in this type of contact, than is gathered in the motion, it is necessary:
either for the rotor to exhibit high flexibility in the radial direction (whilst remaining stiff in the tangential and normal direction), it being possible to achieve this through grooves in the rotor,
or to modify the mode of contact.
There are in fact in the prior art numerous embodiments which exploit discontinuous contact between rotor and stator. In these embodiments, one, two or more vibrating elements, angularly distributed, will be used as stator assembly. Each vibrating element used exhibits an elliptical displacement on its surface owing to an excitation produced as appropriate by two perpendicular exciters (patents SU 681479 and U.S. Pat. No. 4,613,782, the contents of which are incorporated by reference) or aligned exciters (FR 2 709 213, the content of which is incorporated by reference, and U.S. Pat. No. 5,532,540, the content of which is incorporated by reference).
In all these cases, the vibrating elements form stator subassemblies. They are supplied in phase or with a phase shift related to the number of them and to their geometrical distribution (FR 2 709 213) and placed on one and the same fixed frame, but are not mechanically intercoupled.
Finally, there are in the prior art numerous embodiments exploiting discontinuous contact between rotor and stator, but this time without resorting to a two-phase supply. The principle is to use a tangential component of the vibratory velocity, tapped off from the small protuberances linked to an elastic body on which an exciter device generates standing waves. The U.S. Pat. No. 4,882,500, the content of which is incorporated by reference, describes various applications of linear or rotary motors, where the interest stems from employing a single electrical supply pathway to excite an elastic plate. Compared with traveling wave devices, these more economical motors exhibit low performance. The zones of the plate where the elliptical motion is usable actually represent a very small portion thereof.
The aim of the invention is to obviate the aforementioned drawbacks and, more particularly, to eliminate the spurious effects of radial vibrations.
A traveling wave ultrasonic motor is provided that has at least one ring-shaped stator, two groups of electromechanical transducers each having at least one pair of diametrically opposed longitudinal transducers, polarized in opposite directions, disposed perpendicularly to the stator, that is to say parallel to the axis of the motor, distributed around the stator, in permanent contact with the stator and excited by an alternating current with a xcfx80/2 phase shift between the groups so as to produce a traveling wave-like deformation on the surface of the stator, and at least one rotor held elastically in contact with the stator for the rotational driving thereof by the traveling wave produced on the stator.
The invention uses a traveling wave motor stator having longitudinal exciters of which only two or four diametrically opposed contact zones are exploited, per rotor.
The motor according to the invention is one wherein the stator is in contact with the rotor only via at least one pair of equal and diametrically opposed segments whose aggregate span is at most equal to a wavelength of the traveling wave produced in the stator.
The spurious effects of radial vibrations are thus eliminated and the invention makes it possible to exert a larger contact pressure between stator and rotor.
In the U.S. Pat. No. 5,298,829, the contents of which are incorporated by reference, it is proposed, with a different aim, to limit the contact between the stator and the rotor of a piezoelectric motor according to two diametrically opposed toothed segments. Given the mode used, the aggregate span of the bearing zones is appreciably greater than a wavelength so that this limited contact does not have the effect sought by the invention at all.
Furthermore, as far as the present invention is concerned, the introduction of an alternating nature into the mode of contact allows indirect measurement of the speed of the rotor from the stator supply signals alone.